It appears that the main body off text of my original message, that described the issue in detail, was not included in my original post.

That being said, the issue is with any expression (simple or complex) that I add to the opacity control of an Outer Glow layer style.

The error is a warning that the expression had been disabled because of an error in line 1.

Even the basic alt/opt-click expression produces the same error.

Ignoring the warning, I am able to see that the expression is working correctly in the AE visualizer. However, when I export the composition, the final rendered product does not include the effect of the expression.